Re: Finding the server where a PV's IOC is located

[Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov>]

On 11/24/21 5:59 AM, Sobhani, Bayan via Tech-talk wrote:
> Hi Ralph,
> 
> I only have a basic understanding of how the channel access protocol works, but my understanding is that the client just makes a socket connection to a port and then sends the channel access commands. 
> 
> I might be making some incorrect assumptions, but I think that if to get the host from your local machine's perspective you send some channel access commands to a local port, then to get the host from the gateway machine's perspective you can just make a socket connection to the right port on the gateway machine and send the same commands, without needing to ssh in the gateway machine. Do you know if this is how it works?

I *think* what you are describing is analogous to a NAT firewall,
is largely passing packets back and forth verbatim.  CA (and PVA)
gateways are more analogous to a caching HTTP proxy, where data
from one "upstream" (from IOC) subscription is passed along to
multiple "downstream" (to client) subscriptions.

At present I don't think there is no way to remotely query a CA gateway
about the state of upstream subscriptions.


> Alex
> 
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> *From:* Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Ralph Lange via Tech-talk <tech-talk at aps.anl.gov>
> *Sent:* Wednesday, November 24, 2021 2:44:50 AM
> *To:* EPICS Tech Talk <tech-talk at aps.anl.gov>
> *Subject:* Re: Finding the server where a PV's IOC is located
>  
> On Tue, 23 Nov 2021 at 21:41, Sobhani, Bayan via Tech-talk <tech-talk at aps.anl.gov <mailto:tech-talk at aps.anl.gov>> wrote:
> 
>     If you type “cainfo <pv name>”, one of the fields it returns is “host”, which is often the server that the PV’s IOC is running on. ____
> 
>     __ __
> 
>     But sometimes the “host” is really a gateway server and not the IOC server itself. For example:____
> 
>     __ __
> 
>     [...] 
> 
>     __
> 
>     And then to find the real IOC server I would do “ssh cagw01.cs.nsls2.local” and then do “cainfo XF:03ID-CT{RG:C2}T-I” again, and I get:____
> 
>     __ __
> 
>     [...] 
> 
>     __
> 
>     And now I have xf03idc-ioc2-313.nsls2.bnl.local, which is the real IOC server. Is there a way to find this IOC server without ssh?
> 
> 
> Using Channel Access? No.
> Many installations (including NSLS 2 btw.) are using the ChannelFinder directory service to collect and provide that kind of information.
> 
> Cheers,
> ~Ralph
>